How to run neutron metadata agent in an isolated network with enable_isolated_metadata

Issue

When one uses neutron provider networks with an external router, there is no need for L3 agent function in Openstack.

However, the NAT rule for 169.254.169.254 (metadata agent) is done in the virtual router's namespace. How can one run the metadata agent without a neutron router on the subnet?

Cloud-init is unable to set up ssh key for an instance that is using a provider network.
Unable to inject ssh key at boot time
cloud init ssh key